Getting aircraft into the air at short notice. Involves pilots running and shouting alot, then taking off rather bloody quickly.
Practised on a regular basis at RAF Leuchars and Leeming, although rather than intercepting the Luftwaffe or the Red Air Force, the pilots usually spend their time shepparding tactically disorientated aircraft to an airfield or making sure no nasty terrorist has his hands on an airliner.
In the Battle of Britain pilots would be scrambled into their Spitfires and Hurricanes by red telephone and bell, nowadays they sit in a comfy crew room waiting for an alarm bell and teleprinter to tell them to scramble their Typhoons and Tornadoes - after the three hour kit check, two hour roll call and half an hour of a Health and Safety Powerpoint presentation
During the Cold War Bombers were to be scrambled to drop nuclear bombs on the USSR, although practised it was fortunately never required.
Scramble is also known as 'Quick Reaction Alert.'